Optical apparatus such as a projector and a display which use light sources of three wavelengths of R, G, and B that are primary colors are becoming widespread and penetrating. As a light source, generally, a light-emitting diode (LED) is mainly used, but, when used for projecting or displaying an image and a video, a brighter image is strongly required, and thus, a laser (LD) is becoming more popular than an LED.
Compared with an LED, a laser has features of higher light intensity, a narrower wavelength width, and higher coherence. Therefore, although a bright image may be obtained, when a laser is used as a light source of a projector, for example, there is a problem that, due to interference of light scattered by minute unevenness on a screen or the like, noise called speckles appears on the projected image, which tends to degrade the image quality.
Several proposals have been made to reduce these speckles.
One proposal utilizes the principle that the human eye does not sense speckles when a laser beam is temporally varied at high speed.
For example, Patent Document No. 1 proposes a method of reducing speckles by using a diffractive lens element in which a large number of recesses having different depths are formed on a transparent base material for an illumination apparatus and rotating the diffractive lens element. Further, Patent Document No. 2 discloses a despeckle device which uses a system in which an optical retarder providing an odd integer multiple of half-wave retardation is mechanically rotated and vibrated or liquid crystal is actuated.
On the other hand, as means for reducing speckles which does not use such mechanical or electrical actuation, there is a method in which an optical-path difference larger than a coherence length of a laser light source is given in a laser beam.
For example, Patent Document No. 3 discloses a method in which an optical fiber bundle having an optical-path difference is provided in an optical path of a laser beam.
Further, Patent Document Nos. 4 and 5 disclose methods which enable miniaturization by using a passive component. FIG. 12 illustrates an optical apparatus disclosed in Patent Document No. 4. A laser light flux entering from an incident window 121 of a plane parallel plate 120 at a predetermined angle is refracted and is reflected by a first reflection surface 122, and then, is reflected by a second reflection surface 123. Reflection is repeated alternately at these first and second reflection surfaces 122 and 123. Transmission windows (not shown) are partly provided in the first reflection surface 122. By setting an optical-path difference between laser light fluxes passing through these transmission windows to be larger than a coherence length of a laser light source, light fluxes which pass through adjacent transmission windows are made incoherent from each other.
Further, FIG. 13 illustrates an optical system of an illumination apparatus disclosed in Patent Document No. 5. In FIG. 13, a beam splitter 130 includes a parallel plate in which a reflection film is bonded to both surfaces of a quartz plate, and reflectivities of reflection surfaces a, b, c, d, and e are 75%, 66.5%, 50%, 0%, and 100%, respectively. When a laser beam enters the beam splitter 130, four beams having substantially the same intensity are output. An optical-path difference between the respective beams is set to be larger than a coherence length, and thus, the four beams are incoherent from one another. Further, each beam is split into two beams polarized to be perpendicular to each other by a birefringent crystal 131 located at the back. Beams which are polarized to be perpendicular to each other do not interfere with each other, and thus, the eight beams in total do not interfere with one another, that is, the eight beams in total are incoherent from one another.
Further, Patent Document No. 6 discloses a method of reducing speckles by diffusing light with the use of a diffractive optical element so that the diffusion angle of the light becomes larger than that before the incidence in a spatial modulation device for modulating light entering from a light source.